1. Field of the Invention
The present invention relates to a power tool including a direct-current motor as its drive source such as a rotary hammer drill.
2. Description of Related Art
A power tool using a direct-current motor as its drive source has been widely put to practical use. A direct-current motor, which is used as the drive source of a power tool of this type, as shown in FIG. 10, is composed of a yoke set 130 and an armature set 140.
Here, the yoke set 130 is composed of a substantially cylindrical-shaped yoke 131 made of ferromagnetic material such as iron, two or more arc-curve-shaped magnets 132 disposed inside the yoke 131, and a ring-shaped dust guard 133 mounted on the yoke 131 in such a manner that it is fitted with the peripheral edge of the opening of the axial-direction one end portion of the yoke 131.
Also, the armature set 140 is composed of a rotation shaft 104 the two end portions of which are rotatably supported by bearings 105 and 106, and armature 141 and a cooling fan 107 which are both fixed to the rotation shaft 104. When the armature set 140 is rotatably inserted through the yoke set 130, a direct-current motor is assembled.
When incorporating the direct-current motor having the above structure into a two-split housing which is divided in two right and left parts, there is employed a method in which, after the yoke set 130 is stored into one of the two division parts of the housing, the armature set 140 is inserted into the yoke set 130; or, a method in which a direct-current motor composed of the yoke set 130 and armature set 140 previously assembled together is stored into one of the two division parts of the housing, the other division part of the housing is placed on one division part of the housing, and the two division parts are fastened together using a screw or the like, whereby the direct-current motor is incorporated into the housing.
By the way, when material having a weak magnetic force such as ferrite is used as the material of the magnet 132 fixed to the inner peripheral portion of the yoke 131 of the yoke set 130, no problem can occur in an operation for inserting the armature set 140 into the yoke set 130.
However, when material having a strong magnetic force such as a Neodybium-bond-system or rare-earth-system sintered alloy is used as the material of the magnet 132, since the armature 141 is attracted by the magnet 132 because of the strong magnetic force of the magnet 132 when inserting the armature set 140 into the yoke set 130, it is not easy to insert the armature 140 into the yoke set 130; and also, the armature 141, especially, the commutator 141a and winding portion 141b of the armature 141 can be damaged by the magnets 132 and dust guard 133 and thus, during use, there is a possibility that wires can be broken or there can occur a rare short.
In view of the above, as shown in FIG. 11, there is employed an assembling method in which, while a protection cap 142 made of non-magnetic material is left put on the commutator 141a of the armature 141, the armature set 140 is inserted through the inside of the yoke set 130 and, finally, the protection cap 142 is removed from the armature 141. According to this method, the protection cap 142 can prevent the armature 141 from being attracted to and damaged by the magnets 132.
On the other hand, since the two or more magnets 132 are fixed to the inner surface of the yoke 131, the yoke 131 receives the counter torque of the magnetic force for rotating the armature 141 and is thereby going to rotate. Also, as regards the magnets 132 fixed to the inner surface of the yoke 131, their angle positions with respect to a carbon brush are preset, which makes it necessary to position the yoke 131 and to prevent the yoke 131 against rotation.
Therefore, conventionally, there is employed such a positioning and rotation prevention structure as shown in FIG. 12 or 13.
Specifically, FIG. 12 is a transverse section view of the yoke 131. A structure shown in FIG. 12 is used to form width across flats 131b on the outer peripheral portion of the yoke 131 and bring the width across flats 131b into engagement with plane portions formed in a housing (not shown) to thereby attain the positioning and rotation prevention of the yoke 131.
Also, FIG. 13A is a front view of the yoke 131, and FIG. 13B is a side view of the yoke 131. A structure shown in FIG. 13 is used to form a rectangular-groove-shaped notch 131a in part of the axial-direction one end face of the yoke 131 and bring a projecting portion formed in a housing (not shown) into engagement with the notch 131a to thereby attain the positioning and rotation prevention of the yoke 131.
By the way, there has been proposed and already put to practical use a power tool in which, for the purpose of securing the high rigidity of a housing made of resin and for other purposes, the housing is formed in a cylindrical shape and a direct-current motor is incorporated in the barrel portion of the cylindrical-shaped housing (see Japanese Patent NO. 2005-40880).
However, when the assembling method using the protection cap 142 shown in FIG. 11 is applied to the cylindrical-shaped housing, since the yoke set 130 is previously incorporated in the barrel portion of the cylindrical-shaped housing, there arises a problem that, after the armature set 140 is inserted into the inside of the yoke set 130, the protection cap 142 cannot be removed. Therefore, when the armature set 140 is inserted into the yoke set 140, the armature 141 is attracted to the magnets 132, which gives rise to the above-mentioned various inconveniences.
Also, as regards the yoke 131 positioning and rotation preventing structure shown in FIGS. 12 and 13, because, before the yoke set 130 is incorporated into the cylindrical-shaped housing, the positioning and rotation prevention of the yoke 131 are not yet attained, the yoke set 130 must be fitted into the cylindrical-shaped housing while peeping into the inside of the cylindrical-shaped housing. Therefore, it is not easy to incorporate the yoke set 130 into the cylindrical-shaped housing, resulting in the poor efficiency of the assembling operation.